Electromechanical relays

ABSTRACT

A device for preventing welding of the switch contacts of a heavy duty electromechanical relay comprises means (15, 16 and 17) adapted in use for monitoring the voltage supply to the relay coil (2) and detecting low voltage excursions therein. A counter (14) counts the number of low voltage excursions in the voltage supply to the relay coil (2) and an electronic switch (12) disconnects the voltage supply to the relay (2) if the number of low voltage excursions counted exceeds a predetermined number.

The present invention relates to a device for preventing the welding ofthe switch contacts in a heavy duty electromechanical relay. It is ofparticular significance to heavy duty relays used in automotiveapplications where cost considerations preclude the use of materials forthe switch contacts which would give a better mechanical resistance tocontact welding.

In many cases, heavy duty electromagnetic switches use plain copperswitch contacts which have a high tendency to fuse together whenbreaking inductive, high current, loads. In order to prevent this fromoccurring, the switch contactor is frequently fitted with a returnspring which has a high load versus compression factor. This springexerts sufficient force to the moving contactor to break any contactfusing which might occur as the relay is de-energized. Provided that therelay coil is well designed and the coil voltage is close to its designparameter then the contactor usually survives its service life withoutcatastrophic welding. However, particularly in the automotiveenvironment, maintenance of the coil voltage at its design parameter isdifficult because even under normal conditions, the high currentsdemanded by heavy duty motors (e.g. starter motors) and lightingcircuits (e.g. the cold resistance of halogen lamps) can cause lowvoltage excursions to occur. In the case of an electromechanical relaycontrolling the supply of heavy current to a D.C. starter motor, thefollowing situation typically occurs.

The potential difference across the vehicle battery is close to thedesigned coil voltage, typically 12 volts, but due to a high resistancepath in the battery/motor circuit, at the instant that high current isdrawn, the potential across the coil is reduced to a value which isbelow the drop out level of the electromagnetic switch. Consequently,the switch contacts are broken. However, at the instant the switchcontacts are broken, the heavy load current is removed and the potentialdifference across the coil returns to a level at which the switchcontacts are closed. This sequence repeats itself and an oscillatorysituation therefore arises in which the switch contacts are switched inand out at a relatively high frequency. This is sometimes referred to as"doorbelling" or "fizzing". Of course, when this occurs severe arcingand plasma generation occur at the gap between the switch contacts. Thisrapidly increases the temperature of the switch contacts and if thecondition is allowed to persist failure of the electromechanical relaycan occur for any one of the following reasons:

i) switch contacts weld together

ii) plastic insulators break down under the intense heat generated

iii) return and switch contacts springs degrade under the intense heatgenerated.

The high resistance path in the battery/motor circuit which gives riseto this condition can be caused by several factors. For example, a highbattery internal impedence due to undercharging, low or contaminatedacid, sulphation or incorrect specification of battery. It may also becaused by poor installation of wiring, incorrect wire sizes fitted orterminal corrosion.

It is an object of the present invention to provide a device for usewith a heavy duty electro-mechanical relay which obviates or at leastsubstantially mitigates the problem of "doorbelling" or "fizzing"described hereinabove, thereby preventing welding of the switchcontacts.

According to the present invention there is provided a device forpreventing welding of the switch contacts of a heavy dutyelectromechanical relay, which device comprises means adapted in use formonitoring the voltage supply to the relay coil and detecting lowvoltage excursions therein, a counter for counting the number of lowvoltage excursions in the said voltage supply to the relay coil, andswitching means for disconnecting the voltage supply to the relay if thenumber of low voltage excursions counted exceeds a predetermined number.

With the device of the present invention the number of times the relayswitch contacts are allowed to oscillate is reduced to a very low number(the preset number set by the counter), thus "doorbelling" or "fizzing"is minimised and welding of the switch contacts is prevented.

The switching means conveniently takes the form of an electronic switch,such as a MOSFET transistor.

Preferably, the said means for monitoring and detecting low voltageexcursions in the condition of the voltage supply to the relay coil isable to disregard transient low voltage excursions. Conveniently, thisis achieved through the use of an RC filter network with a suitable timeconstant.

Advantageously, the device of the present invention further comprises athermal sensor which is adapted in use to be mounted in the terminalhousing of a relay to detect over heating thereof. This enables a highresistance fault condition at the switch contact, such as may, forexample, occur if the terminals become loosened or corroded, to bedetected.

Preferably, the device further comprises means for preventing operationof the relay when the battery supply is below a preset level.

The device may also comprise indicator means for indicating a faultcondition.

An embodiment of the present invention will now be described, by way ofexample, with reference to the circuit diagram shown in the accompanyingdrawing.

In the circuit diagram there is shown a heavy duty electromechanicalrelay, generally designated by reference 1, comprising a relay coil 2and switch contacts 3. As shown the relay 1 controls the supply ofcurrent from a battery (Vbat) to a D.C. starter motor 4.

Within the housing of the switch contacts 3 of the relay 1 is a thermalmonitoring device 5, which takes the form of a thermistor, but couldalso be a temperature sensing semiconductor. This is used to detect thetemperature of the housing as described below and remove the power tothe relay coil 2 if over-heating of the switch contacts 3 occurs.

The thermistor 5 is connected at one end to a reference voltage (Vref)and at the other end to a resistor 6 which in turn is connected toground. A potential divider is thus formed at the mid point between thethermistor 5 and the resistor 6. As the temperature of the thermistor 5increases, its resistance decreases (negative temperature coefficient)and the voltage at the mid point (Vt) increases. This voltage ismonitored by an analogue comparator 7 which compares the mid pointvoltage (Vt) with a reference voltage (Vref1). The value of thisreference voltage (Vref1) is set such that it corresponds to the midpoint voltage (Vt) produced when the thermistor temperature is at avalue deemed to be the maximum allowable temperature of the contacthousing. Thus when the contact housing approaches an overheatedcondition, the output of the comparator 7 switches state.

The output of comparator 7 is monitored by one of the three inputs of athree input "OR" gate 8 which creates a high logic-level output when theoutput of the comparator 7 is switched. The output of the "OR" gate 8 isfed into a low pass RC filter network consisting of a capacitor 9 and aresistor 10. This filters out any transient noise signals which may begenerated by the automotive supply. Any signal which has a pulseduration which is longer than the time constant of the RC filter networkthen triggers a 10 second timer 11. When this timer 11 is triggered itsoutput changes state from a high logic to a low logic level. The outputof the timer 11 normally supplies a high logic level to a power MOSFETtransistor 12 which is used to provide current to the relay coil 2, butremoves this power when the gate voltage is removed, due to triggeringof the timer 11. The timer 11 remains triggered while its trigger inputis at the high logic level.

A second comparator 13 provides protection against low voltage operationof the relay call 2. This comparator 13 monitors the vehicle batterysupply (Vbat) by comparing a fraction of the incoming voltage, (providedby a potential divider chain of resistors 14, 15 and 16) with areference voltage Vref2. A transient suppressor 17 protects the input ofthe comparator 13 from any high voltage transients generated by thevehicle battery supply. If the battery voltage falls to a value deemedto be too low for the correct operation of the relay coil 2, then thecomparator output changes from a low level to a high level state. Inthis situation, the output of the comparator 13 removes the power to therelay coil 2 by feeding the second input of the three input "OR" gate 8and triggering the associated circuitry comprising the timer 11 asdescribed hereinbefore for the high temperature cut out.

Since the relay 1 is intended for use in automotive applications, theprotection circuit must ignore the inevitable, transitory, low voltageexcursions produced when the D.C. starter motor 4 is first switched on.The low pass RC filter network provided by resistor 10 and capacitor 9prevents the protection circuitry from false triggering when thissituation occurs (the pulse length of a transient is shorter than thetime constant of the filter components R and C).

In order to prevent extended contact oscillation which can cause severecontact erosion or welding, additional circuitry is included whichdetects this condition and removes the power to the relay coil 2.

A five-stage Johnson counter/divider 14 is clocked each time the switchcontacts 3 are closed and then opened. This clock signal is provided bysensing the voltage on the output terminal of the switch contactor 3.The signal is conditioned by a resistor 15 and a transient suppressor 16to remove any high voltage spikes and is further filtered by a signalfilter 17 which provides a suitable clocking signal for the Johnsoncounter 14. The Johnson counter 14 is normally permanently reset.However, when the relay coil 2 is powered by closing switch 30, a resetsignal is applied to the Johnson counter 14, from the battery (Vbat) viaa resistor 18, transient suppressor 19 and a two input "OR" gate 20.Therefore, while the relay coil 2 is operated, the Johnson counter 14 isclocked by any oscillations of the switch contacts 3 which are severeenough to actually break the connection between the switch contacts. Ifthe contacts oscillate five times, then the output of the Johnsoncounter 14 changes its logic state and triggers the third input of thethree input "OR" gate 8 and then the input of the timer 11 via the lowpass RC filter network 9, 10. Thus the power is removed from the relaycoil 2 and is inhibited from further operation for ten seconds. Theconstruction of the switch contacts 3 is such that they can safelytolerate five oscillations within a ten second periled and therefore thecircuit prevents the severe damage which would occur (i.e. severe arcingor welding ) if the switch contacts 3 were allowed to oscillate underload indefinitely, as is the case with standard switch contacts.

When the power to the relay coil 2 is removed, the Johnson counter 14 isreset.

Two power up reset circuits 21, 22 are used to set up the ten secondtimer 11 and Johnson counter 14 to their "normal" states when power isfirst applied to the circuit.

A diode 23 and a resistor 24 suppress the negative high-voltage spikescreated across the relay coil 2 when power is removed.

What is claimed:
 1. A device for preventing welding of the switchcontacts of a relay caused by oscillation of the switch contacts due tolow voltage excursions in the voltage across the relay coil, comprisinga counter having a clock input which is clocked each time the switchcontacts open and close following the connection of a voltage supply tothe relay coil, and switching means for disconnecting the voltage supplyto the relay when the clock count exceeds a predetermined number.
 2. Adevice according to claim 1 wherein the switching means takes the formof an electronic switch.
 3. A device according to claim 2, wherein theelectronic switch comprises a MOSFET transistor.
 4. A device accordingto claim 1, wherein the counter is reset each time power is applied tothe relay coil.
 5. A device according to claim 1, wherein the clockinput is connected to one of the switch contacts via a signalconditioning circuit.
 6. A device according to claim 1, comprising atimer device which inhibits further operation of the relay for apredetermined period following operation of the switching means todisconnect the relay coil from the voltage supply.